The invention provides a low-dopant polycrystalline silicon chunk.
On the industrial scale, crude silicon is obtained by the reduction of silicon dioxide with carbon in a light arc furnace at temperatures of about 2000° C.
This affords “metallurgical grade” silicon (Simg) having a purity of about 98-99%.
For applications in photovoltaics and in microelectronics, the metallurgical grade silicon has to be purified. For this purpose, it is reacted, for example, with gaseous hydrogen chloride at 300-350° C. in a fluidized bed reactor to give a silicon-containing gas, for example trichlorosilane. This is followed by distillation steps in order to purify the silicon-containing gas.
This high-purity silicon-containing gas then serves as a starting material for the production of high-purity polycrystalline silicon.
The polycrystalline silicon, often also called polysilicon for short, is typically produced by means of the Siemens process. This involves heating thin filament rods of silicon by direct passage of current in a bell-shaped reactor (“Siemens reactor”), and introducing a reaction gas comprising a silicon-containing component and hydrogen.
The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiHnX4-n (n=0, 1, 2, 3; X=Cl, Br, I). It is preferably a chlorosilane, more preferably trichlorosilane. Predominantly SiH4 or SiHCl3 (trichlorosilane, TCS) is used in a mixture with hydrogen.
In the Siemens process, the filament rods are typically inserted perpendicularly into electrodes present at the reactor base, through which they are connected to the power supply. Every two filament rods are coupled via a horizontal bridge (likewise composed of silicon) and form a support body for the silicon deposition. The bridge coupling produces the typical U shape of the carrier bodies, which are also called thin rods.
High-purity polysilicon is deposited on the heated rods and the bridge, as a result of which the rod diameter grows with time (CVD=Chemical Vapor Deposition/gas phase deposition).
After the deposition has ended, these polysilicon rods are typically processed further by means of mechanical processing to give fragments of different size classes, classified, optionally subjected to a wet-chemical purification and finally packed.
Polycrystalline silicon serves as a starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or CZ process) or by means of zone melting (float zone or FZ process).
More particularly, however, polycrystalline silicon is required for production of mono- or multicrystalline silicon by means of pulling or casting processes, this mono- or multicrystalline silicon serving for production of solar cells for photovoltaics.
Since the quality demands on polysilicon are becoming ever higher, constant process improvements are necessary with respect to contamination with metals or dopants. A distinction should be made between contamination in bulk and contamination at the surface of the polysilicon fragments or rod pieces.
US2003/0159647 A1 discloses polycrystalline silicon chips having contamination of less than or equal to 0.06 ppba of boron and of less than or equal to 0.02 ppba of phosphorus in the bulk. US 2003/0159647 A1 does not give any information about the contamination of the surface with dopants.
EP1544167 A1 discloses polycrystalline silicon granules, the particles having a particle size between 100 μm and 3000 μm, a dopant content of phosphorus less than 300 ppta, a dopant content of boron less than 300 ppta and a total content of the metals Fe, Cr, Ni, Cu, Ti, Zn and Na of less than 50 ppbw. EP1544167 A1 does not give any information about the contamination of the surface of the granules with dopants.
Both publications cited mention only dopant concentrations in the bulk (US2003/0159647 A1) or total concentrations (bulk and surface, EP1544167 A1).
It is known that the process steps for production of polysilicon, such as the comminution of rods, have an influence on the surface contamination with metals and dopants.
DE 41 37 521 A1 describes a process for analyzing the concentration of contaminants in silicon particles, which comprises adding particulate silicon to a silicon vessel, processing the particulate silicon and the silicon vessel to give monocrystalline silicon in a float zone, and determining the concentration of contaminants present in the monocrystalline silicon. The concentrations of boron, phosphorus, aluminum and carbon in the silicon vessel used were determined and give a reproducible background value.
The values of boron, phosphorus and carbon obtained by means of FTIR (Fourier Transform IR Spectroscopy) after the float zone process were corrected by the proportion which originated from the silicon vessel.
This application also shows that the fragmentation of a polycrystalline silicon rod leads to contamination of the silicon. This is possible by introducing silicon fragments into the silicon vessel, subjecting it to the float zone process and then analyzing it for contaminants. Since the contamination of the base material prior to fragmentation is known, the additional contamination as a result of the fragmentation can be concluded.
DE 43 30 598 A1 likewise discloses a process which enables conclusion of the contamination of silicon as a result of comminution processes. A silicon block was crushed to lumps. The silicon lumps were then subjected to a zone melting process and converted to a single crystal. A wafer was sawn out of the single crystal and analyzed for boron and phosphorus by means of photoluminescence. Compared to the average boron and phosphorus contents of the silicon block used, an increase in the boron and phosphorus concentrations is found, which is attributable to the comminution process among other factors.
According to SEMI MF 1398, dopants are analyzed by means of photoluminescence in an FZ single crystal obtained from the polycrystalline material (SEMI MF 1723). As an alternative, low-temperature FTIR is used (SEMI MF 1630). Fundamentals of the FZ process are described, for example, in DE 3007377 A.
In the FZ process, a polycrystalline stock rod is gradually melted with the aid of a high-frequency coil, and the molten material is converted to a single crystal by seeding with a monocrystalline seed crystal and subsequent recrystallization. In the course of recrystallization, the diameter of the single crystal forming is first increased in a cone shape (cone formation) until a desired final diameter has been attained (rod formation). In the cone formation phase, the single crystal can also be mechanically supported in order to take the load off the thin seed crystal.
In the prior art, efforts have been made to examine the influence of a single process step on any surface contamination of polysilicon with dopants.
However, it has to date not been possible to distinctly reduce the dopants at the surface of polysilicon, even though it is known that dopants affect the physical properties of the material.
The problems described gave rise to the objective of the invention.